The invention relates to the field of Pre-Press or Graphic Arts that involves the preparation of images for print. This industry nowadays usually employs digital computers and it processes images coded as files of digital data. These image files usually fall into two categories, raster files and vector files. Raster files describe an image as a two dimensional array of picture elements called pixels. A pixel contains the attribute of image color at its particular location within the image. Usually all pixel comprising an image are assumed to be rectangular, and of the same dimensions. Vector files describe an image in terms of various geometrical shapes such as a line segment or a polygon. The specification of these shapes contains the attributes of color of each shape, and its dimensions. A well-known page description language (PDL) language comprising vector description of images is Postscript.
It is a common practice in the industry to convert images between raster and vector formats. The process of conversion of a file from Postscript to a raster format is often called RIP (Raster Image Processing). The process of conversion from raster to Postscript is herein called Ceps2Ps.
The target of a Pre-Press image processing activity is, by definition, the conversion of the image from digital form to an analogue form of ink or toner on paper or other substrate. A printing press is a common device for printing ink on paper, and so it is often stands at the end of a Pre-Press activity. The press often produces large sheets of paper on each of which several distinct images are printed. Post-press activity is then often required to cut, fold and bind these sheets of paper to form printed booklets. One such large sheet of paper can be turned by this Post-press activity to a booklet of several pages each containing one of these distinct images as the printed content of the page.
It is a known by-product of the Post-press process that the distinct images get rotated one in respect to the other by a small angle. When a press produces a sheet of paper with several distinct images that are not rotated one in respect to the other, they will appear rotated in the resulting booklet. This byproduct of the Post-press process is sometimes and herein called ‘bottling’. It is a common practice to correct the image orientation using the Pre-press process to correct for bottling that is expected to occur at Post-press, so that correct orientation would be observed for the booklets. The image processing function of rotation is generally well known and commonly used in the art.
It, thus, may happen that an image coded in some raster format needs to be passed through the following Pre-press process. At a first step, the raster image is converted to Postscript format. Then, at a second step, the Postscript coded image is rotated by a small angle. Finally, at a third step, a RIP operation produces raster pixels for printing.
It is common for an image to contain straight line segment elements that are either precisely horizontal or precisely vertical, i.e. line segments that exactly parallel to either horizontal or vertical edges of the page on which the are printed. Thus its is common for the second step of rotation to produce line segments that form a small angle to either vertical or horizontal direction. These segments are called typical bottling segments' herein.
An inherent and common behavior of the RIP has been observed when it converts typical bottling segments to raster format as follows. Undesirable artifacts are produced that appear as cracks within the image and between typical bottling segments.
It is not desirable to prevent the appearance of the undesirable artifacts by introducing changes in the behavior of the RIP, because a corrected RIP would behave differently than other RIP products in the industry, and it is required that all RIP products within the industry behave in a similar fashion. Otherwise the industry will prefer to avoid using the Postscript format altogether.
The present invention seeks to prevent the appearance of the undesirable artifacts by introducing changes to the Ceps2Ps operation.
The invention is suitable for a particular Ceps2Ps method of operation as follows. Ceps2ps divides a raster image into segments of like color. Each segment is then described as a geometrical shape such as a polygon in the syntax of Postscript. The invention seeks to eliminate the undesirable effect of the appearance of cracks, or gaps between these segments.
Appendix A lists sample Postscript code generated by a Ceps2Ps function. This code contains examples of some polygon image segments, and in this sample they are rectangular.
A full description of the language is given, for example, in “PostScript Language Reference Manual”, Second Edition, by Adobe System Incorporated, Addison-Wesley Publishing Company, ISBN 0-210-18127-4, herein named The Red Book.
Note that lines begin with the character ‘%’ are comments in the language. Such lines do not translate to any image elements. Some comment lines are used herein to mark areas of interest within the Postscript code. Comments are described in Page 27 in Section 3.2.2 in The Red Book.
Under the line “% item 1” there is listed a first example of an image element composed of three rectangles. Note the use of the “setcmyktcolor” command explained in Page 496 in The Red Book, and the use of the “rectfill” command explained in Page 473 in The Red Book. Roughly explained, “setcmykcolor” specifies a color composed of cyan, magenta, yellow and black ink, and “rectfill” fills a rectangular shape with that color. Note that the first rectangle has no transparent colors at all, the second rectangle has one transparent color, and the third has two transparent colors.
Under the line “% item 2” there is listed a second example of an image element composed of two rectangles. Note that they are smaller in size relative to the rectangles described above.